COSMOS, VOL. II - World eBook Library

I)] DISCOVERIES IN THE CELESTIAL SPACES. G97
being of opinion that some were higher
and others lower than
the rest. The idea formed of the heaven of the fixed stars
was extended to the planets and thus arose the ;
theory of the
eccentric intercalated spheres of Eudoxus and Menoechmus, and
of Aristotle, who was the inventor of retrograde spheres. The
theory of epicycles,
a construction which adapted itself most
readily to the representation and calculation of the planetary
movements, was, a century afterwards, made by the acute
mind of Apollonius to supersede solid spheres. However much
I may incline to mere ideal abstraction, I here refrain from
attempting to decide historically whether, as Ideler believes,
it was not until after the establishment of the Alexandrian
Museum that " a free movement of the planets in space was
regarded as possible," or whether before that period the
intercalated transparent spheres t(of which there were twentyseven
according to Eudoxus, and fifty-five according to Aris-
totle), as well as the epicycles which passed from Hipparchus
and Ptolemy to the middle ages, were regarded generally not
as solid bodies of material thickness, but merely as ideal
abstractions. It is more certain that in the middle of the
sixteenth century, when the theory of the seventy-seven hoinocentric
spheres of the learned writer, Girolamo Fracas-
toro, found general approval ; and when, at a later period,
the opponents of Copernicus sought all means of upholding
the Ptolemaic system, the idea of the existence of solid
spheres, circles, and epicycles, which was especially favoured
by the Fathers of the Church, was still very widely diffused.
Tycho Brahe expressly
boasts that his considerations on
the orbits of comets first proved the impossibility of solid
spheres, and thus destroyed the artificial fabrics. He filled
the free space of heaven with air, and even believed that
the resisting medium when disturbed by the revolving
heavenly bodies, might generate tones. The un-imaginative
Rothmann believed it necessary to refute this renewed
Pythagorean myth of celestial harmony.
Kepler's great discovery that all the planets move round the
sun in
ellipses, and that the sun lies in one of the foci of these
ellipses, at length freed the original Copernican system from
eccentric circles and all epicycles.* The planetary structure
* A better insight into the free movement of bodies, and into the independence
of the direction once giveu to the earth's axis, and into the